Over the last four decades, the density of integrated circuits has increased by a relation known as Moore's law. Stated simply, Moore's law says that the number of transistors on integrated circuits (ICs) doubles approximately every 18 months. Thus, as long as the semiconductor industry can continue to uphold this simple “law,” ICs double in speed and power approximately every 18 months. In large part, this remarkable increase in the speed and power of ICs has ushered in the dawn of today's information age.
Unlike laws of nature, which hold true regardless of mankind's activities, Moore's law only holds true only so long as innovators overcome the technological challenges associated with it. One of the advances that continues to allow Moore's law to hold true is optical overlay metrology, which allows extremely accurate overlay measurements to be performed between patterned layers. For example, optical overlay metrology can measure how accurately a first patterned layer aligns with respect to a second patterned layer above or below the first patterned layer. If a workpiece contains layers that are out of alignment, then the substrate can be re-worked (e.g., a first or second layer which is photoresist can be stripped) and returned to the photolithography process to be exposed again at a corrected alignment. Unfortunately, the tolerances or measurement uncertainties of these metrology techniques are too large for next generation devices.